No, I don't think they are necessarily ignorant. If they say they understand that they understand the increased risks with narcosis in an emergency situation when exceeding a density of 6.2 g/L and accept those risks, I'm good with that. I'm not good with people who dismiss the science. I have had a conversation with one individual who dismissed Dr. Mitchell as "just an occasional diver" who trusts the divers at his former dive center who "regularly" dive to 70 meters or more on air. A friend of mine in Europe is friends with a number of people who died diving that way. And they don't switch to trimix until 100 meters. Now obviously there is a big gap between 40 and 100 meters on air. However, the risks increase dramatically above a gas density of 6.2 g/L. That's ultimately my point. It does come down to risk assessment, and we all assume different levels of risk. But I'll never understand or respect those who dismiss science.
The problem I have with the gas density argument for 100' swapping is that "the data says the risk starts increasing significantly" around there, but there is no stated level of risk associated with it defined as far as I can see AND the data supporting that comes assuming that not only are you breathing that gas at the stated depths, but also doing fairly extreme exertion at the same time (all the test data that came up with the 8.5 kPa number, Warkander et al, for a high risk of incapacitation that I can find were using divers doing significant exercise for extended periods of time, a situation I've never been in while diving personally). Let's assume that you're at 130' on air, your buddy is at 130' on 21/35, and you both have a burst LP hose. What is the basic risk of a bad outcome using air assuming you're trained (10-45 seconds of activity to deal with the issue and then to start your ascent)? 0.000001%? 0.00001%? I mean if it's 100 more risky on air than trimix, but the $100 in trimix is going to change your risk of a bad outcome from 1 in 1,000,000 to 1 in 100,000 is it worth an extra $400 in gas every weekend for to do 4 dives, especially if you know that your dive isn't likely to be in a condition where you're doing the exercise levels that the testing was generally done in?
It would be safer for me to drive an armored vehicle, as the chance that a road rage incident etc. would be survivable would be significantly higher, but it's not cost effective to me based on the low probability of a negative outcome in the first place (despite it being significantly safer if that low probability event happens).
Now, if the science said "if you have a problem at 110' on air, there's a 5% chance you'll end up dying but if you were on 21/35 instead you'd have a 0.005% chance of dying" it would be a no-brainer to say it's worth it. That, as far as I can tell, has never been the message from anyone. Simply "it'll reduce the risk of X significantly", where X is a measurable item that doesn't necessarily equate to a specific negative overall outcome (i.e. death etc.) happening, but no statements or conclusions about what that risk of an actual bad outcome is mentioned in any of the things I've read or watched. If such an assessment exists I'd love to see it, but what I've seen is we have two general camps: those who say "the risk is really low on air so they're happy on air to a point", and those who say "the risk is lower on trimix so I'm going to use trimix regardless of how potentially safe it is without trimix". Often the first group omits the "to a point" and the second group omits the "regardless of how potentially safe it is without trimix" as each have decided that their person choice of "safe" is what defines "safe".
Usually, both camps ignore any other factors that impact risk. Personally, I'll dive air to 150-160' at my local quarry, with few if any entanglement hazards, no current to speak of, no overhead environments to be concerned with, navigation can always consist of "go up", etc. as the added expense of going to trimix for such a relatively safe dive doesn't seem worth it (especially with the vaguely defined benefit it would offer based on gas density and the conditional benefit from reduced narcosis). Now, if I were to be penetrating the Oriskany hanger at 176' in open water with currents and an overhead environment and all the other risk factors, the benefits of reduced narcosis start to make the dive seem significantly safer and the reduced gas density, when I could potentially encounter strong currents where strong exercise might be necessary provides another compelling reason to think that the benefits would be worth it.
So, for me, it comes down to relative risk. With actual risk being relative from person to person and dive to dive, and the science not giving a definitive answer as to what is "safe" to dive and what is "unsafe" to dive, I evaluate based on the dives I'm doing, the information I have, and the risks I'm willing to take. Which is what I suggest everyone do, which means we're likely to all come up with different answers as to what we consider "safe".
